Malawian President Bingu wa Mutharika has died after suffering a heart attack, medical sources have told Al Jazeera, although his death has not been officially announced.

The 78-year-old was rushed to hospital in the capital Lilongwe after collapsing on Thursday morning , and state media subsequently reported that he had been flown to South Africa for emergency treatment.

The Reuters news agency described chaotic scenes at the hospital as Mutharika's wife, Calista, and senior cabinet ministers left the building, amid concerns that the president's death could prompt political upheaval.

The constitution makes clear that Joyce Banda, Mutharika's vice-president, is first in line to take over as head of state in the southern African nation, although her expulsion from the ruling party in 2010 could complicate the transition.

Such a situation would appear to put her on a collision course with Mutharika's inner circle, including Peter Mutharika, the country's foreign minister and the president's brother who was groomed to take over.

But Bright Sonani, a Malawian journalist with the Nation newspaper, said that whatever differences exist between Mutharika and Banda, the constitution remains supreme.

He said "it is clear-cut" that the vice-president has to take over when the president is incapacitated or dead.

Sonani said local media were still waiting for an official announcement, adding that the authorities would not normally announce news of the president's death in a hurry as they have to prepare the country for the sad news.

Police on Thursday were deployed across the country's capital, while 15 army officers took up positions around Banda's residence, witnesses said.

Aid problems

Mutharika, a former World Bank economist, came to power in 2004 and presided over a seven-year economic boom, underpinned by foreign aid and some favourable rains, that made Malawi one of the world's fastest-growing countries.

Economic growth, however, came to a halt last year, after a dispute with the United Kingdom led to diplomatic expulsions in both countries and the freezing of millions of dollars in aid to Malawi.

The aid suspension led to an acute shortage of dollars in the Malawian economy, hampering fuel, food and medicine imports.

Mutharika's diplomatic isolation and economic plight worsened in July 2011 when the US shelved a $350m programme to overhaul the country's dilapidated power grid, after police killed at least 19 people in a crackdown on an unprecedented wave of anti-government protests.